Abstract UMTS-WLAN heterogeneous mobile networks allow a single mobile user with different radio technologies to access different mobile networks, but how to secure such interworking networks and provide a seamless service is a new challenge. Even if EAP-AKA protocol provides authentication services in UMTS-WLAN interworking networks, a fast re-authentication of EAP-AKA protocol still cannot overcome high re-authentication delays and delay-sensitive applications. Because a mobile user is authenticated by a remote RADIUS or a HLR/HSS both resided in 3G-UMTS home networks whatever a full authentication or a fast re-authentication is occurred. It causes that huge re-authentication session loads and cryptographic operation loads concentrated on the RADIUS and the HLR/HSS. In addition, such an inefficient authentication/re-authentication protocol also causes long authentication/re-authentication latency. Therefore, this article proposes a novel protocol named fast iterative localized re-authentication (FIL re-authentication) to replace the fast re-authentication of EAP-AKA protocol. The proposed protocol not only has minor modifications to attain the same security level as EAP-AKA, but it uses both localized re-authentication process and iterative process within the AP to handle the fast re-authentication locally and iteratively for speeding up the re-authentication. Additionally, the IEEE 802.11 WLAN simulation mode based on Network Simulator 2 is used for proving a valid implementation and for analyzing the performance of the proposed protocol. It shows superior results in comparison to the existing EAP-AKA protocol.